The SCM plugins I have used in the past have had the ability to support variable expansion:

On a master I can define say VERSION:

VERSION=1.2.2

I can pass it as a parameter to a slave job using SVN for use in a branch specification:

http://svn.corp/project/branches/${VERSION}

The only place it's maintained is on the master (good when I'm running half a dozen architecture specific slaves).

The Gerrit Repo plugin has the ability to use a URL as a manifest (instead of a fixed manifest in the 'Local Manifest' field). Unfortunately I'm unable to pass a URL as a variable:

Master:
JOBNAME=mybuild
JOBNUM=12

Slave (in 'Local Manifest'):

http://server/${JOBNAME}/${JOBNUM}/default.xml

It does not expand, neither do any of the other fields... Bummer!